1. Field of the Invention
The present invention relates to an acoustic filter used in wireless communication devices, and more particularly, to a film bulk acoustic resonator (hereinafter, referred to as “FBAR”) which implements a high pass filter for passing only a specified high frequency component, and a method for manufacturing the same.
2. Description of the Related Art
As mobile communication devices, such as mobile phones have become popular, a small and light filter for such devices has become increasingly in demand. In the meantime, as a means for implementing the small and light filter, an FBAR has been introduced. The FBAR can be produced in bulk at a very low cost, and manufactured in a very small size. In addition, the FBAR enables a high quality factor value which is a special feature of the filter, and can be used in a micro frequency band. In particular, the FBAR is able to realize even a personal communication system (PCS) band and a digital cordless system (DCS) band.
In general, an FBAR element comprises a laminated resonance part created by a first electrode, a piezoelectric layer and a second electrode vapor-deposited in the above order on a substrate. The FBAR is operated as follows. Electric energy is applied to an electrode, and an electric field which temporally changes is induced in the piezoelectric layer. Then, the electric field causes a bulk acoustic wave in the piezoelectric layer in the same direction as a vibration in the laminated resonance part, and generates the resonance.
The FBAR element includes, as shown in FIGS. 1A through 1D, a Bragg reflector-type FBAR and an air gap-type FBAR.
The Bragg reflector-type FBAR of FIG. 1A is formed by vapor-depositing in order of a reflection layer 11, a lower electrode 12, a piezoelectric layer 13, and an upper electrode 14. Here, the reflection layer 11 is formed by vapor-depositing on a substrate 10 materials having a large difference of elastic impedance in an alternate manner. Thus-structured Bragg reflection-type FBAR elements can effectively generate the resonance since all elastic acoustic wave energy passed through the piezoelectric layer 13 is not transferred to the substrate 10, but reflected at the reflection layer 11. The Bragg reflector-type FBAR has a firm structure without a stress from bending, however, it is hard to form the reflection layer of at least 4 layers in precise thickness for total reflection. Additionally, a significant amount of manufacturing time and a large cost are required.
On the other hand, the air gap-type FBAR uses an air gap instead of the reflection layer to separate the substrate from the resonance part, and is divided into several types according to the manufacturing method used. Different types of air gap-type FBAR elements are illustrated in FIGS. 1B through 1D.
The FBAR element in FIG. 1B is a bulk micro-machined FBAR fabricated in a manner that a membrane 21 is formed by SiO2, for example, on the substrate 20, a cavity part 23 is defined by the anisotropic etching of a rear side of the substrate 20, and the acoustic resonator 22 is formed on the membrane 21. An FBAR element thus structured is not practical due to its very weak structure and a low recovery rate.
The FBAR element in FIG. 1C is a surface micro-machined FBAR fabricated as follows. A sacrifice layer (not shown) is formed on the substrate 30, and an insulation membrane 32 is formed on the sacrifice layer and the substrate 30. A first electrode 33, a piezoelectric layer 34 and a second electrode 35 are vapor-deposited in order, and finally, the sacrifice layer is removed to form an air gap 31. More specifically, a via hole (not shown) is formed to connect the exterior of the element to the sacrifice layer inside the element, and an etchant is injected through the via hole to remove the sacrifice layer. Consequently, the air gap 31 is formed. Furthermore, in manufacturing the membrane, the sacrifice layer needs to be slanted, which causes a weak structure due to a high remaining stress of the membrane.
The FBAR element of FIG. 1D is fabricated in the following manner. A cavity part 45 is defined by etching a substrate 40 using a photo-resist membrane, and a sacrifice layer (not shown) is vapor-deposited on the cavity part 45. A membrane 41, a first electrode 42, a piezoelectric layer 43, and a second electrode 44 are vapor-deposited in order on the sacrifice layer and the substrate 40. Then, an air gap 45 is formed by etching the sacrifice layer. In the above manufacturing method, a wet etching and a dry etching are employed in forming the air gap 45. In case of wet etching, it is hard to remove the etchant, moreover, when the etchant is not completely removed, the element becomes weak due to continuous actions of the etchant, and the resonance frequency may be changed. In case of dry etching, on the other hand, the etching is accomplished by a plasmatic gas. At this time, physical impact can be caused by an ion and a molecule, and the membrane 41 or the substrate 40 can be deteriorated by high temperature.